1. Field of the Invention
This invention relates to membranes for molecular hydrogen (H2) gas separation from a gas mixture. In one aspect, this invention relates to metallic membranes for H2 separation. In another aspect, this invention relates to metallic membranes for H2 separation at high temperatures. In another aspect, this invention relates to supported metallic membranes for H2 separation. In yet another aspect, this invention relates to metallic membranes disposed directly on and supported by hollow fiber membranes. In another aspect, this invention relates to metallic membranes supported on polymeric hollow fiber membranes.
2. Description of Related Art
Solid hydrocarbon fuels such as coal and biomass are converted to gaseous fuels at high temperatures by partial oxidation with air and/or steam. Exemplary of such conversions are processes taught by U.S. Pat. Nos. 4,057,402 and 4,369,045 (coal gasification) and U.S. Pat. Nos. 4,592,762 and 4,699,632 (biomass gasification). Synthesis gases produced by these processes comprise primarily hydrogen and carbon monoxide, typically with a hydrogen/CO molar ratio in the range of about 0.6 to about 6.0. Because of the abundance of solid hydrocarbon fuels, they are potentially major sources of hydrogen, particularly if cost effective means for extracting the hydrogen from the gaseous fuel products can be devised.
Gasification of solid hydrocarbon fuels is carried out at high temperatures in the range of about 600° C. to about 1400° C. Although these temperatures favor the kinetics of chemical reactions, materials selection for use in hydrogen separation is often limited to ceramics.
Hydrogen forming reaction systems such as steam/methane reforming wherein methane and water are reacted to form carbon dioxide and hydrogen and water-gas-shift reaction systems wherein carbon monoxide is reacted with water to form carbon dioxide and hydrogen are also well known in the art. Steam/methane reforming is used as a catalytic reaction system for the production of hydrogen. Conventional catalytic systems for steam/methane reforming require catalytic reaction temperatures on the order of 1800° F. followed by purification processes including acid gas removal and hydrogen purification. The water-gas-shift reaction is frequently used following gasification of naturally occurring carbonaceous materials, such as coal, peat, oil shale, and the like, wherein the product gas temperatures must be lowered to about 750° F. to drive the water-gas-shift reaction.
Membranes have been used to recover or isolate a variety of gases, including hydrogen, helium, oxygen, nitrogen, carbon monoxide, carbon dioxide, water vapor, hydrogen sulfide, ammonia, and/or light hydrocarbons. Applications of interest include the separation of hydrogen from gas mixtures containing gases such as nitrogen, carbon monoxide, carbon dioxide, and/or light hydrocarbons in addition to hydrogen.
Such membrane separations are based on the relative permeability of two or more gaseous components through the membrane. To separate a gas mixture into two portions, one richer and one leaner in at least one gaseous component, the mixture is brought into contact with one side of a semi-permeable membrane through which at least one of the gaseous components selectively permeates. A gaseous component which selectively permeates through the membrane passes through the membrane more rapidly than at least one other gaseous component of the gas mixture. The gas mixture is thereby separated into a stream which is enriched in the selectively permeating gaseous component or components and a stream which is depleted in the selectively permeating gaseous component or components.
The use of metallic foils, films or membranes for separating gaseous mixtures, particularly hydrogen, is well known. See, for example, U.S. Pat. No. 1,174,631, which teaches hydrogen separation from a gaseous mixture using a thin sheet or film of platinum or palladium at a temperature preferably above about 1470° F., and U.S. Pat. No. 2,773,561, which teaches purification of hydrogen by permeation through a thin film of silver-palladium under a pressure differential between opposite sides of the film. H2 permeates through metals under a solubility/diffusion mechanism with pressure differential providing the driving force. However, while a thin film of metal is desirable to minimize the diffusion path, such thin metal films require a support to withstand high pressure differentials. In addition, higher temperatures also increase the permeability of H2 through the metal membrane.
Membranes for gas separation also have been fabricated from a wide variety of polymeric materials, including cellulose esters, polyimides, polyaramids, and polysulfones. An ideal gas separation membrane is characterized by the ability to operate under high temperature and/or pressure while possessing a high gas separation factor (selectivity) and high gas permeability. The problem is finding membrane materials which possess all the desired characteristics. Polymers possessing high gas separation factors generally have low gas permeabilities, while those polymers possessing high gas permeabilities generally have low gas separation factors. In the past, a choice between a high separation factor and a high gas permeability has been unavoidably necessary. Furthermore, some of the membrane materials previously used suffer from the disadvantage of poor performance under high operating temperatures and pressures.
Conventional H2 permeable hollow fiber membranes are polymeric and are used, for example, in ammonia production. Hollow fiber membranes are a preferred membrane configuration for gas separation applications because of their high surface area/volume ratio. However, these membrane materials do not provide the near perfect selectivity to only H2 permeating through the membrane barrier as obtained by metal membranes.